1. Field of the Invention
The present invention relates generally to printers and, more particularly, to a method and apparatus for ensuring good electrical contact between interconnect pads on a print cartridge and the corresponding interconnect pads in the stall of a print carriage.
2. Related Art
Inkjet printheads operate by ejecting a droplet of ink through a nozzle and onto a recording medium, such as a sheet of paper. When a number of nozzles are arranged in a pattern, such as a rectangular matrix, the properly sequenced ejection of ink from each nozzle causes characters or other images to be printed on the paper as a printhead is moved relative to the paper. The printhead is usually part of a disposable print cartridge containing a supply of ink. The print cartridge is designed for easy installation and removal from a stall in a print carriage. Print cartridges are installed and removed hundreds of times over the life of a print carriage.
In one type of thermal inkjet print cartridge, the print cartridge includes: 1) an ink reservoir and ink channels to supply ink proximate to each of the nozzles; 2) a printhead in which the nozzles are formed in a desired pattern; 3) a substrate attached to a bottom surface of the printhead, a series of thin film heater resistors being formed on the substrate, generally one resistor below each nozzle and 4) interconnect pads formed on an insulating tape with which electrical connections are made to corresponding interconnect pads on the print carriage.
To print a dot of ink from a nozzle, an electrical current is passed through paired interconnect pads of the print carriage and the print cartridge to a selected resistor of the print cartridge. The heater is ohmically heated, in turn heating a thin layer of adjacent ink. This results in vaporization of the ink, vapor bubbles in the ink causing a droplet of ink to be ejected through an associated nozzle onto the paper. The resistors in the substrate are connected by conductors formed on the insulating tape to interconnect pads on the insulating tape. The interconnect pads, the conductors and the insulating tape are collectively known as the TAB circuit, since the insulating tape is bonded to the printhead by the well-known tape automated bonding (TAB) process.
There are several problems associated with the prior art devices that result in inadequate electrical contact between corresponding interconnect pads. In the prior art, the interconnect pads of the print carriage were terminal points of a circuit formed on a flexible insulating tape (also known as a "flex" circuit). Previously, the flexible insulating tape was mounted on the print carriage so that the interconnect area was over-constrained. FIG. 1 is a schematic of a cross-sectional view of a flexible insulating tape 87 in which two opposite ends 91 and 92 are attached to print carriage 30.
One reason for inadequate electrical contact between interconnect pads is that, with multiple sides attached to the print carriage 30, the flexible insulating tape 87 is overconstrained causing non-uniform deflection of the tape 87 when a contact force F is applied to the tape 87. As shown in FIG. 1, the flexible insulating tape 87 buckles when the contact force F is applied. Buckling results in inadequate contact between some of the interconnect pads of the print carriage and the print cartridge since not all of the interconnect pads on the tape 87 are deflected the same amount.
Another reason for inadequate electrical contact between corresponding interconnect pads is the need for each interconnect pad of print cartridge 24, 25, 26 or 27 to be positioned precisely with respect to each interconnect pad in the carriage stall of print carriage 30. Inadequate positioning of corresponding interconnect pads due to non-uniformity in height of the interconnect pads (henceforth also "flatness" problem) may result in "missing dots" due to inadequate contact. In the prior art, the flex circuit had bumps on one side and dimples on the other side. The interconnect pads were formed on the bumps of the flex circuit. The flex circuit was supported by an elastomeric pad that had columns on opposing sides.
One prior art elastomeric pad is described in U.S. Pat. No. 4,706,097 to Harmon. As shown in FIG. 3A of U.S. Pat. No. 4,706,097 to Harmon, tips of columns of the elastomeric pad facing the flex circuit are inserted into the dimples on the flex circuit. The columns of the elastomeric pad act to push the interconnect pads of the flex circuit into contact with corresponding interconnect pads of the TAB circuit. Because of the deformability of the elastomeric material, columns of the elastomeric pad also act to compensate for localized minor variations in height of the interconnect pads on the flex circuit or the TAB circuit.
One problem with the prior art elastomeric pad is that the height of the columns on the side opposite the side facing the flex circuit that is necessary to ensure adequate contact force results in long column buckling or bending of the columns. Long column buckling results in inadequate contact between corresponding interconnect pads since a bent column does not exert the necessary minimum contact force.
Another problem with the prior art elastomeric pad is that the spring characteristics of the columns require tight control of the relative positions of the print cartridge and the print carriage. Tight control is necessary because a small variation in displacement (i.e., change in relative positions of the print carriage and print cartridge) results in a large variation in contact force.
Also, as shown in FIG. 2 of U.S. Pat. No. 4,706,097 to Harmon, a relatively large variation of displacement delta, .DELTA. results in large variation in load L.sub.1 between the interconnect pads. If the flex circuit interconnect pad is displaced too far, the load may become great enough to damage the interconnect pads. On the other hand, if the displacement drops below delta .DELTA., the load drops below L.sub.1 resulting in inadequate electrical contact between the interconnect pads of the flex circuit and TAB circuit.
Moreover, in order to ensure proper electrical contact, the print cartridge must be positioned in the print carriage so that the corresponding interconnect pads on the flex circuit and TAB circuit are positioned in parallel planes. If the print cartridge is aligned at an angle with respect to the print carriage, there is a wide variation in contact forces between some pairs of interconnect pads. Consequently, some interconnect pads may be damaged, or there may be inadequate electrical contact between some pairs of pads. The prior art elastomeric pad was unable to compensate for such misalignment.
Also, in order to have proper contact between the interconnect pads it is necessary for each print cartridge 24-27 and each carriage stall to be relatively clean. Presence of residual hot melt, dried ink, package shavings or small fibers can result in contamination failures. Any contamination, such as a 3 mil diameter piece of skin, caught between the interconnect pads results in improper contact which results in the "missing dots" problem. In the prior art, to ensure clean surfaces, a cleaning brush or a Q-tip application was used to brush away the contaminants. The drawback with this technique is that the Q-tip application itself left fibers which in turn caused contamination failures of the interconnect pads.
Reliability of contact between interconnect pads can also be improved by increasing the force of contact between the interconnect pads. However, there are several problems associated with increasing the contact force in the prior art device. For example, a large increase in contact force may damage the interconnect pads on the print carriage. Also, if the print cartridge is inserted at an angle, the farthest interconnect pads are subjected to a greater force so that the maximum load was limited to what the farthest interconnect pads can withstand. Another problem is that since the interconnect pads of the print carriage are formed on a flexible insulating tape supported by an elastomeric pad that has bumps, increasing the contact force results in buckling of the bumps of the elastomeric pad.
Furthermore, in the prior art, when the print cartridge was inserted into the print carriage, a small radius rotary motion between the print cartridge and print carriage was used to bring the corresponding interconnect pads into contact with each other. The prior art rotary motion is described in detail in U.S. Pat. No. 4,872,026 to Rasmussen et al.
Finally, if the properties of the elastomeric pad were changed to solve one of the above problems, such a change adversely affected the other problems so that all the problems could not be addressed simultaneously by the prior art elastomeric pad.
Thus, there is a need for an inexpensive and reliable method and structure for improving the electrical contact between the interconnect pads on a print cartridge and the corresponding interconnect pads in the stall of a print carriage.